Range finding with laser light beams is often used to detect stationary objects in land survey applications, at a distance of about 1 meter up to 4 km. Because of the narrow spatial spreading of a laser beam on propagation, relatively small and distant objects can be selectively illuminated. The range finding process starts with observing the object with a telescope and pointing to it by overlaying the image with a reticule in the eyepiece of the telescope. The laser emitter is precisely aligned on the optical axis of the telescope, by a process known as “boresighting”. This guarantees that the laser light will be incident on the place indicated by the centre of the reticule and that sufficient reflected laser light can be captured. The laser measurement is activated (normally by pushing a button) and subsequently the range is displayed. By scanning the laser over the surface of the target, under supervision of a signal processor, 2D and 3D maps of the object can be realized, provided that the resolution of the ranging method is substantially smaller than the size of the object. This scanning process is typically mechanical since the emitted laser beam and the optical axis of the telescope have to remain aligned at all times. A further drawback of such mechanical systems is a limited operational life time.
A recent general overview of range finding methods can be found in the article ‘Optical methods for distance and displacement measurements’, G. Berkovic, E. Shafir, Advances in Optics and Photonics 4, 441-471 (2012). Broadly speaking, three methods as described below in more detail are discussed in this article.
A first very common method for distance measurement is the “time of flight” method, where the distance is found by sending electromagnetic waves (e.g., light) to the object and measuring the time taken for the waves to travel from the sensor to the object and back. A laser pulse of a few nanoseconds duration is sent to a target, while simultaneously a fast digital counter is started. The scattered radiation returns to the sender and is there detected by a fast photo detector which stops the counter.
The disadvantage of this method is that a short, powerful laser pulse is needed, in order for the amplitude of the echo to be higher than the noise level. Ranges up to about 100 meter require peak powers of 10-50 Watts, whereas ranges up to 5 km ask for 10-1000 kilowatts. The pulse length is typically a few nanoseconds.
A second method is phase modulation, wherein the outgoing light is sinusoidal amplitude-modulated with a constant frequency, and the phase difference between the local wave and the received wave is measured. The phase difference can be obtained by measuring the time differences between the zero crossings of the emitted wave and the reflected wave. In both cases, the maximum distance that can be measured, while avoiding ambiguities, is determined by the selected modulation frequency. This maximum range is less than 50 meters for commercial models.
A third method is the FMCW or frequency modulation continuous wave method. This is a form of coherent detection. In the phase shift method, the phase difference between incident and reflected field is detected. In the FMWC method, the frequency difference is detected. The emitted beam of a wavelength tuneable diode laser is periodically and linearly chirped in frequency (saw tooth envelope). The received signal has the same frequency profile, but time-shifted by the roundtrip time. The emitted and reflected signals are mixed in a square-law photo detector, whose main AC component is the frequency difference of both signals. This frequency, together with the chirp duration and the range of the frequency sweep, determine the roundtrip time and hence the distance to the target.
Technologically, this is the most demanding method, since the wavelength of the laser must be configurable, which is far from obvious. This technique is used predominantly for metrological applications, where the form of a surface must be measured remotely and accurately in 3D. Examples are the body of an airplane or the hull of a ship. The disadvantages here are thus the cost and the complexity of the system.
Furthermore, all three methods as mentioned above are basically meant for point measurements by observing and aiming to a fixed point, with a telescope part of the range finder. Consequently, prior observation by an operator is necessary.
In U.S. Patent Application Publication No. 2008/0018881, a homodyne detection scheme for linear FM modulated lidar is presented in which pulse de-chirping is performed in the optical domain. This homodyne detection scheme comprises a laser, a modulator, a telescope, a balanced photo detector, processing circuitry and a waveform generator. In the method using this system, both the optical signal and the local oscillator are modulated by the same linear frequency chirp. The de-chirping process is accomplished within the photodiode. In order to measure the distance to an object, the beat frequency is measured and that beat frequency is calibrated in a distance.
The disadvantage of this system is that it is not possible to scan a surface therewith. Only single range point measurements can be performed.
Accordingly, there is a need to resolve the above-identified shortcomings of the existing systems. More particularly, there is a need to provide an improved detection system that can scan a surface in a simple, cost efficient and very fast way, and furthermore to provide such a detection system wherein no prior observation by an operator has to be done.